By ion-exchange technique, it is possible to produce optical waveguides into a glass surface by replacing ions originally in the glass (typically sodium ions, i.e. Na.sup.+ ions) by ions (such as K.sup.+, Ag.sup.+, Cs.sup.+, Rb.sup.+, Li.sup.+ or Tl.sup.+ ions) that increase the refractive index locally. Optical waveguide structures are patterned by thin-film technique and photolithography on some insulating or metallic film forming a so-called ion exchange mask for an ion exchange between the glass substrate and some ion source. As ion sources are often used salt melts, but in the case of Ag-Na ion exchange a thin silver film can also be used as an ion source. By using different ions, it is possible to produce waveguides with very different properties. For elementary knowledge of an ion exchange process, reference is made to the article [1] "Ion-exchanged glass Waveguides: A review", R. V. Ramaswamy, Journal of Lightware Technology, Vol. 6., No. Jun. 6, 1988.
In the following, description is primarily focussed on processes in which Na.sup.+ ions are replaced either by Ag.sup.+ or K.sup.+ ions (Ag-Na ion exchange or K-Na ion exchange). In K-Na ion-exchanged channels the greatest possible refractive index increase is rather small (about 0,01), and therefore, K-Na ion exchange suits well for the production of optical waveguides compatible with a single-mode optical fiber. On the other hand, due to the small difference between the refractive indices of the optical waveguide and the surrounding glass substrate, the field distribution of light in K-Na ion-exchanged optical waveguides is always rather wide. This leads to a poor coupling with e.g. a laser diode having a very narrow (about 2 .mu.m) field distribution of light. On the other hand, the width of the field distribution of an optical single-mode fiber is about 10 .mu.m. K-Na ion exchange causes stresses on the glass, which leads to double-refraction in optical waveguides produced in this way. This can be utilized in optical components separating polarizations, but double-refraction is also a drawback, for instance in wavelength selective optical components.
By Ag-Na ion exchange again, a considerably larger refractive index increase (about 0,1) can be achieved, which makes it possible to produce optical waveguides with a considerably better compatibility with laser diodes. Additionally, thanks to the larger refractive index, smaller radii of curvature can be used in optical waveguide structures without light escaping at bends of an optical waveguide out of the optical waveguide. On the other hand, if a large refractive index difference is used in optical waveguides, the efficiency of coupling with an optical fiber is poor. Differently from K-Na ion-exchanged optical waveguides, Ag-Na ion-exchanged optical waveguides are usually not double-refractive.
It would be useful in many applications of integrated optics, if optical waveguides with very different properties could be combined in one and the same component structure. It would, for instance, be advantageous to provide a component one end of which has a good coupling with an optical fiber and the other end a good coupling with a laser diode. In the case of ion-exchange technique, this would be effected by coupling K-Na ion-exchanged and Ag-Na ion-exchanged optical waveguides on the same glass substrate. However, an adaptor device of some kind is required for coupling together two different optical waveguides inside the component with as small losses as possible.